The present invention relates to the forming or wire section of a machine for producing a web of paper, or the like from a fibrous stock suspension and more particularly to a stationary support member for the endless drainage wire belt.
Stationary supporting members are used for support of the endless wire belt on which a fibrous stock web is formed from the fibrous suspension flowing continuously onto the wire belt. In addition, the doctor-like leading edge of the head board of the supporting member is also used to carry away the white water which has flowed out of the forming fibrous stock web through the meshes of the wire belt and clings to the underside of the wire belt. At the same time, due to the inclination of the drainage area of the head board relative to the direction of travel of the wire on the underside of the endless wire belt, a vacuum is generated which intensifies drainage. The intensity of this drainage depends on the size of the angle of inclination of the drainage area.
In paper machines on which the operating conditions change frequently, for example, changes of paper grade, operating speed, etc., a variation of that angle of inclination at the stationary supporting members is frequently necessary. For this reason, a reliable design has for a long time been sought for the adjustment of the angle of inclination. The following publications describe such designs:
1. U.S. Pat. No. 2,928,465 PA1 2. U.S. Pat. No. 3,027,940 PA1 3. U.S. Pat. No. 3,201,308 PA1 4. U.S. Pat. No. 3,337,236 PA1 5. U.S. Pat. No. 3,497,420 PA1 6. U.S. Pat. No. 3,520,775 PA1 7. U.S. Pat. No. 3,535,201 PA1 8. U.S. Pat. No. 3,647,620 PA1 9. DE-OS No. 25 10 492 PA1 10. DE-GM No. 78 07 296 PA1 11. CH-PS No. 601 554 PA1 12. EP-OS No. 00 80 447
There are many reasons that, in practice, nonadjustable stationary supporting members are used almost exclusively. If it is desired to vary the angle of inclination of the drainage area on these nonadjustable supporting members, the head board must be removed from the supporting member and thus from the paper machine, and must be replaced by another head board with a different angle of inclination. This method is unsatisfactory, because it is very dangerous to carry out such a head board change while the paper machine is running. Furthermore, it necessitates the exchangeability of the head board of such a nonadjustable supporting member so that the head board rests on the supporting member with a certain clearance. This clearance may be the cause of undesired changes in the angle of inclination of the drainage area.
On many of the well-known supporting members with an adjustable angle of inclination, there is the risk that the supporting member will tend to vibrate, precisely because of the adjustability of the angle of inclination of the drainage area. The vibrations also caused by this in the wire belt disturb the uniform drainage of the fibrous stock web. This danger exists above all when the entire supporting member can be swivelled to vary its angle of inclination, that is, it rests in swivel supports at both its ends, i.e. on the tending and drive sides of the paper machine. A further difficulty consists in keeping the adjusting member for the angle of inclination of the drainage area free from contamination. This is important because the adjusting member becomes, on the one hand, difficult to move as a result of contamination and, on the other hand, the required angle of inclination cannot be set with the desired accuracy and reproducibility.
Many of the well-known designs have the disadvantage that they require a great deal of space, so that it is not possible to make the distance between two consecutive supporting members as small as would be necessary to achieve optimum drainage. This applies particularly to modern paper machines of large width (the order of 6-10 m).
In the case of a number of other well-known designs, the head board contacting the underside of the wire belt is not made of a stiff, hard material, but instead is made of a flexible, deformable material. This enables the change in the angle of inclination of the drainage area to be made by deforming the head board. Practice has, however, shown that only a head board constructed as a completely rigid body (made of ceramic or a similar hard material) ensures the required high service life and stability of shape. Flexible deformability of the head board has the additional disadvantage that the drainage area is not substantially flat, but becomes at least in part, rounded. Observations have shown that this produces non-uniform drainage across the width of the paper machine, in a similar way to the support of the wire by means of table rolls.
The above-mentioned publication No. 2; U.S. Pat. No. 3,027,940, describes a stationary supporting member with a deformable head board. FIGS. 6 to 8 therein disclose a supporting structure for the head board, which comprises two beams with C-shaped cross-section extending transversely across the machine width, namely a C-beam secured rigidly in the machine and a movable C-beam. Viewed in cross-section, the arms of the two C-beams engage in each other so that the top arm of the rigid C-beam is located between the arms of the movable C-beam and, consequently, the bottom arm of the movable C-beam is between the arms of the rigid C-beam. The movable C-beam is coupled to the rigid C-beam by means of a hinge element which is formed as a spring plate. Furthermore, several adjustable stops distributed over the length of the supporting member are fitted to the rigid C-beam, on which the movable C-beam can be supported against the rigid C-beam. Finally, a clamping element formed as an inflatable hose is provided between the top arm of the rigid C-beam and the bottom arm of the movable C-beam. The supply of a pressure means to this hose, to deform the head board by a certain dimension, can press the movable C-beam onto the stop surfaces and thereby clamp the two C-beams with each other. The head board is secured at its front end (i.e. near the leading edge contacting the wire belt) to the C-beam by means of a hinge. At its rear end, the head board is secured to the rear end of the above-mentioned spring plate. This kind of fastening has the disadvantage that the rear end of the head board can swing up and down.
A further disadvantage of the well-known design is that the large number of stops, which are formed as bolts distributed over the length of the supporting member, have to be adjusted individually if the angle of inclination of the drainage area is to be varied. It is therefore very difficult or perhaps even impossible to set a selected definite and certain angle of inclination with the desired accuracy while the paper machine is running. On the whole, however, the use of this well-known design is ruled out in practice, because, as already mentioned, the head board is deformable instead of rigid and thus the drainage area is rounded (with a variable radius of curvature) instead of being substantially flat.
In contrast to U.S. Pat. No. 3,027,940, No. 8, U.S. Pat. No. 3,647,620 discloses a stationary supporting member on which the head board is formed as a rigid body with a flat drainage area and is rigidly connected to a movable beam. Furthermore, there is a beam that is rigidly secured in the machine. All these three elements extend across the entire paper machine transversely of the direction of wire travel. The movable beam and the head board can be jointly swivelled because they are coupled to the rigid beam by means of a spring plate which forms a hinge. The spring plate can extend substantially over the entire length of the supporting member or be subdivided into individual sections.
This well-known design has the following disadvantages. The pairs of stop surfaces used to adjust the angle of inclination are formed by spigots 17, which engage in oblique oblong holes of an adjusting board that is slidable in the longitudinal direction. Since there must always be a certain, even if small, clearance between the side walls of the oblong holes and those spigots, this well-known adjusting member necessarily has a certain hysteresis. Furthermore, because of this clearance, vibrations of the head board and of the supporting board relative to the rigid beam, and undesired variations in the angle of inclination of the drainage area, can be expected. Finally, the adjusting unit is arranged unprotected at the rear side of the supporting member, so there is the risk of the above-mentioned danger of contamination.